Do not check XmlDocs unless required

[DedSec256]

Currently, XmlDoc check is performed even if the corresponding diagnostic (3390) is not enabled in the .fsproj.

This PR suggests adding an option to DiagnosticOptions to not call analysis if not required.

[T-Gro]

Looks good in general, but it would be good to cover the functionality with tests covering the positive and negative cases.

[DedSec256]

@T-Gro, not sure if tests are needed here as this PR does not change the visible behavior in any way. The problem that this PR fixes is that checking XML docs is always done, even if its results are not required and therefore ignored. This PR allows analysis to be performed only when its results are required (i.e. warnon:3390 option is enabled in .fsproj).

[T-Gro]

@T-Gro, not sure if tests are needed here as this PR does not change the visible behavior in any way. The problem that this PR fixes is that checking XML docs is always done, even if its results are not required and therefore ignored. This PR allows analysis to be performed only when its results are required (i.e. warnon:3390 option is enabled in .fsproj).

That is what I meant - if the warnon is enabled, a warning should still be produced.

[DedSec256]

There are a lot of tests for XmlDocs checking in
https://github.com/dotnet/fsharp/blob/main/tests/FSharp.Compiler.ComponentTests/Language/XmlComments.fs

with enabled 3390 diagnostic

fsharp/tests/FSharp.Test.Utilities/Compiler.fs

Lines 433 to 435 in 33f9cf8

	/// Turns on checks that check integrity of XML doc comments
	let withXmlCommentChecking (cUnit: CompilationUnit) : CompilationUnit =
	withOptionsHelper [ "--warnon:3390" ] "withXmlCommentChecking is only supported for F#" cUnit

I will add one test that checks for the absence of warnings with disabled diagnostic